A lens is the part of the camera that actually forms the image which is later captured on film or the sensor.
Lenses are described by their "focal length", which is, essentially, the distance from the lens to where it forms an image of an object at infinity. Think of holding a magnifying glass (a "lens") in the sun over a flat surface (ntn-flammable,please!) and moving it back and forth until the hot spot's at its smallest. The distance between the lens and the hot spot is that lens' focal length. Shorter focal length lenses will give you a wider "field of view" than longer lenses. The field of view is just what it sounds like - it's how wide an area you can see when looking through it. The trade-off is that a lens with a wide field of view (a "wide angle" lens) won't magnify a subject as much as a lens with a narrow field of view (a "telephoto" lens) when used from the same place. But for landscapes, you usually want a wide angle lens. For portraits, you usually want a "normal" (in between wide angle and telephoto) or slight telephoto lens. The 18-55 on the 350D (a DSLR, BTW, not DSRL) will meet both those needs adequately while you get started.
f/2.8 or f/5.6 are called f/stops. They measure how "fast" a lens is - how well it collects light. The number is the value you get when you divide the focal length (f) by the lens' opening diameter (aperture). A smaller number f/stop (a "larger" f/stop) means the lens is "faster", because you can use a faster shutter speed with that lens than with one with a smaller aperture. You'll sometimes see a lens listed as f/3.5-5.6. This means that the zoom lens (the only ones you'll see with this type of designation) has a different f/stop at it's shortest and longest focal lengths. For instance, the 18-55 mm is an f/3.5-5.6. At 18 mm, it has an aperture of f/3.5, but at 55 mm, the aperture is only f/5.6.
